When war broke out in Europe in September 1939, the Japanese, despite a series of victorious battles, had still not brought their war in China to an end: on the one hand, the Japanese strategists had made no plans to cope with the guerrilla warfare pursued by the Chinese; on the other, the Japanese commanders in the field often disregarded the orders of the supreme command at the Imperial headquarters and occupied more Chinese territory than they had been ordered to take. Half of the Japanese Army was thus still tied down in China when the commitment of Great Britain and France to war against Germany opened up the prospect of wider conquests for Japan in Southeast Asia and in the Pacific. Japan’s military ventures in China proper were consequently restricted rather more severely henceforth.

In September 1940 Imperial Japan signed the Tripartite Pact with Nazi Germany and fascist Italy. This alliance gave Japanese leaders the security they needed to expand their designs for an East Asian empire into Indochina and beyond. Their ambitions brought them into conflict with the United States, a conflict that erupted into war with the attack on Pearl Harbor in December 1941.Encyclopædia Britannica, Inc.

The German victories over the Netherlands and France in the summer of 1940 further encouraged the Japanese premier, Prince Konoe, to look southward at those defeated powers’ colonies and also, of course, at the British and U.S. positions in the Far East. The island archipelago of the Dutch East Indies (now Indonesia) along with French Indochina and British-held Malaya contained raw materials (tin, rubber, petroleum) that were essential to Japan’s industrial economy, and if Japan could seize these regions and incorporate them into the empire, it could make itself virtually self-sufficient economically and thus become the dominant power in the Pacific Ocean. Since Great Britain, single-handedly, was confronting the might of the Axis in Europe, the Japanese strategists had to reckon, primarily, with the opposition of the United States to their plans for territorial aggrandizement. When Japanese troops entered northern Indochina in September 1940 (in pursuance of an agreement extorted in August from the Vichy government of France), the United States uttered a protest. Germany and Italy, by contrast, recognized Japan as the leading power in the Far East by concluding with it the Tripartite, or Axis, Pact of September 27, 1940: negotiated by Japanese foreign minister Matsuoka Yosuke, the pact pledged its signatories to come to one another’s help in the event of an attack “by a power not already engaged in war.” Japan also concluded a neutrality pact with the U.S.S.R. on April 13, 1941.

On July 2, 1941, the Imperial Conference decided to press the Japanese advance southward even at the risk of war with Great Britain and the United States; and this policy was pursued even when Matsuoka was relieved of office a fortnight later. On July 26, in pursuance of a new agreement with Vichy France, Japanese forces began to occupy bases in southern Indochina.

This time the United States reacted vigorously, not only freezing Japanese assets under U.S. control but also imposing an embargo on supplies of oil to Japan. Dismay at the embargo drove the Japanese naval command, which had hitherto been more moderate than the army, into collusion with the army’s extremism. When negotiations with the Dutch of Indonesia for an alternative supply of oil produced no satisfaction, the Imperial Conference on September 6, at the high command’s insistence, decided that war must be undertaken against the United States and Great Britain unless an understanding with the United States could be reached in a few weeks’ time.

General Tōjō Hideki, who succeeded Konoe as premier in mid-October 1941, continued the already desperate talks. The United States, however, persisted in making demands that Japan could not concede: renunciation of the Tripartite Pact (which would have left Japan diplomatically isolated); the withdrawal of Japanese troops from China and from Southeast Asia (a humiliating retreat from an overt commitment of four years’ standing); and an open-door regime for trade in China. When Cordell Hull, the U.S. secretary of state, on November 26, 1941, sent an abrupt note to the Japanese bluntly requiring them to evacuate China and Indochina and to recognize no Chinese regime other than that of Chiang Kai-shek, the Japanese could see no point in continuing the talks. (See Sidebar: Pearl Harbor and the “Back Door to War” Theory.)

Since peace with the United States seemed impossible, Japan set in motion its plans for war, which would now necessarily be waged not only against the United States but also against Great Britain (the existing war effort of which depended on U.S. support and the Far Eastern colonies of which lay within the orbit of the projected Japanese expansion) and against the Dutch East Indies (the oil of which was essential to Japanese enterprises, even apart from geopolitical considerations).

The evolving Japanese military strategy was based on the peculiar geography of the Pacific Ocean and on the relative weakness and unpreparedness of the Allied military presence in that ocean. The western half of the Pacific is dotted with many islands, large and small, while the eastern half of the ocean is, with the exception of the Hawaiian Islands, almost devoid of landmasses (and hence of usable bases). The British, French, American, and Dutch military forces in the entire Pacific region west of Hawaii amounted to only about 350,000 troops, most of them lacking combat experience and being of disparate nationalities. Allied air power in the Pacific was weak and consisted mostly of obsolete planes. If the Japanese, with their large, well-equipped armies that had been battle-hardened in China, could quickly launch coordinated attacks from their existing bases on certain Japanese-mandated Pacific islands, on Formosa (Taiwan), and from Japan itself, they could overwhelm the Allied forces, overrun the entire western Pacific Ocean as well as Southeast Asia, and then develop those areas’ resources to their own military-industrial advantage. If successful in their campaigns, the Japanese planned to establish a strongly fortified defensive perimeter extending from Burma in the west to the southern rim of the Dutch East Indies and northern New Guinea in the south and sweeping around to the Gilbert and Marshall islands in the southeast and east. The Japanese believed that any American and British counteroffensives against this perimeter could be repelled, after which those nations would eventually seek a negotiated peace that would allow Japan to keep her newly won empire.

Until the end of 1940 the Japanese strategists had assumed that any new war to be waged would be against a single enemy. When it became clear, in 1941, that the British and the Dutch as well as the Americans must be attacked, a new and daring war plan was successfully sponsored by the commander in chief of the Combined Fleet, Admiral Yamamoto Isoroku.

Yamamoto’s plan prescribed two operations, together involving the whole strength of his navy, which was composed of the following ships: 10 battleships, six regular aircraft carriers, four auxiliary carriers, 18 heavy cruisers, 20 light cruisers, 112 destroyers, 65 submarines, and 2,274 combat planes. The first operation, to which all six regular aircraft carriers, two battleships, three cruisers, and 11 destroyers were allocated, was to be a surprise attack, scheduled for December 7 (December 8 by Japanese time), on the main U.S. Pacific Fleet in its base at Pearl Harbor in the Hawaiian Islands. The rest of the Japanese Navy was to support the army in the “Southern Operation”: 11 infantry divisions and seven tank regiments, assisted by 795 combat planes, were to undertake two drives, one from Formosa through the Philippines, the other from French Indochina and Hainan Island through Malaya, so as to converge on the Dutch East Indies, with a view to the capture of Java as the culmination of a campaign of 150 days—during which, moreover, Wake Island, Guam, the Gilbert Islands, and Burma should also have been secured as outer bastions, besides Hong Kong.

In accordance with Yamamoto’s plan, the aircraft carrier strike force commanded by Admiral Nagumo Chuichi sailed eastward undetected by any U.S. reconnaissance until it had reached a point 275 miles north of Hawaii. From there, on Sunday, December 7, 1941, a total of about 360 aircraft, composed of dive-bombers, torpedo bombers, and a few fighters, was launched in two waves in the early morning at the giant U.S. naval base at Pearl Harbor. The base at that time was accommodating 70 U.S. fighting ships, 24 auxiliaries, and some 300 planes. The Americans were taken completely by surprise, and all eight battleships in the harbour were hit (though six were eventually repaired and returned to service); three cruisers, three destroyers, a minelayer, and other vessels were damaged; more than 180 aircraft were destroyed and others damaged (most while parked at airfields); and more than 2,330 troops were killed and over 1,140 wounded. Japanese losses were comparatively small. The Japanese attack failed in one crucial respect, however; the Pacific Fleet’s three aircraft carriers were at sea at the time of the attack and escaped harm, and these were to become the nucleus of the United States’ incipient naval defense in the Pacific. Pearl Harbor’s shore installations and oil-storage facilities also escaped damage. The Pearl Harbor attack, unannounced beforehand by the Japanese as it was, unified the American public and swept away any remaining support for American neutrality in the war. On December 8 the U.S. Congress declared war on Japan with only one dissenting vote.

Pearl Harbor, HawaiiFord Island in Pearl Harbor, Hawaii, as seen from a Japanese aircraft during the attack on the U.S. Pacific Fleet, December 7, 1941.National Archives, Washington, D.C.

On December 7, 1941, Japanese airplanes strike the U.S. naval base at Pearl Harbor, bringing the United States into World War II. From “The Second World War: Triumph of the Axis” (1963), a documentary by Encyclopædia Britannica Educational Corporation.Encyclopædia Britannica, Inc.

Roosevelt, Franklin D.U.S. Pres. Franklin D. Roosevelt denouncing the surprise attack on Pearl Harbor on December 7, 1941—“a date which will live in infamy”—and asking the U.S. Congress to declare war on the Japanese empire.Stock footage courtesy The WPA Film Library

On the day of the attack, December 8 by local time, Formosa-based Japanese bombers struck Clark and Iba airfields in the Philippines, destroying more than 50 percent of the U.S. Army’s Far East aircraft; and, two days later, further raids destroyed not only more U.S. fighters but also Cavite Naval Yard, likewise in the Philippines. Part of the U.S. Asiatic Fleet, however, had already gone south in November; and the surviving major ships and bomber aircraft, which were vulnerable for lack of fighter protection, were withdrawn in the next fortnight to safety in bases in Java and Australia.

Japanese forces began to land on the island of Luzon in the Philippines on December 10. The main assault, consisting of the bulk of one division, was made at Lingayen Gulf, 100 miles north-northwest of Manila, on December 22, and a second large landing took place south of Manila two days later. Manila itself fell unopposed to the Japanese on January 2, 1942, but by that time the U.S. and Filipino forces under General Douglas MacArthur were ready to hold Bataan Peninsula (across the bay from Manila) and Corregidor Island (in the bay). The Japanese attack on Bataan was halted initially, but it was reinforced in the following eight weeks. MacArthur was ordered to Australia on March 11, leaving Bataan’s defense to Lieutenant General Jonathan M. Wainwright. The latter and his men surrendered on April 9; Corregidor fell in the night of May 5–6; and the southern Philippines capitulated three days later.

Japanese bombers had already destroyed British air power at Hong Kong on December 8, 1941, and the British and Canadian defenders surrendered to the ground attack from the Kowloon Peninsula (the nearest mainland) on December 25. To secure their flank while pushing southward into Malaya, the Japanese also occupied Bangkok on December 9 and Victoria Point in southernmost Burma on December 16. The Japanese landings in Malaya, from December 8 onward, accompanied as they were by air strikes, overwhelmed the small Australian and Indian forces; and the British battleship Prince of Wales and the battle cruiser Repulse, sailing from Singapore to cut Japanese communications, were sunk by Japanese aircraft on December 10. By the end of January 1942, two Japanese divisions, with air and armoured support, had occupied all Malaya except Singapore Island. In Burma, meanwhile, other Japanese troops had taken Moulmein and were approaching Rangoon and Mandalay.

On the eastern perimeter of the war zone, the Japanese had bombed Wake Island on December 8, attempted to capture it on December 11, and achieved a landing on December 23, quickly subduing the garrison. Guam had already fallen on December 10. Having also occupied Makin and Tarawa in the Gilbert Islands in the first days of the war, the Japanese successfully attacked Rabaul, the strategic base on New Britain (now part of Papua New Guinea), on January 23, 1942.

A unified American–British–Dutch–Australian Command, ABDACOM, under Wavell, responsible for holding Malaya, Sumatra, Java, and the approaches to Australia, became operative on January 15, 1942; but the Japanese had already begun their advance on the oil-rich Dutch East Indies. They occupied Kuching (December 17), Brunei Bay (January 6), and Jesselton (January 11), on the northern coast of Borneo, as well as Tarakan Island (off northeastern Borneo) and points on Celebes. Balikpapan (on Borneo’s east coast) and Kendari (in southeastern Celebes) fell to the Japanese on January 24, 1942, Amboina on February 4, Makasar City (in southwestern Celebes) on February 8, and Bandjarmasin (in southern Borneo) on February 16. Bali was invaded on February 18, and by February 24 the Japanese were also in possession of Timor.

Meanwhile, on February 8 and 9, three Japanese divisions had landed on Singapore Island; and on February 15 they forced the 90,000-strong British, Australian, and Indian garrison there, under Lieutenant General A.E. Percival, to surrender. Singapore was the major British base in the Pacific and had been regarded as unassailable due to its strong seaward defenses. The Japanese took it with comparative ease by advancing down the Malay Peninsula and then assaulting the base’s landward side, which the British had left inadequately defended. On February 13, moreover, Japanese paratroopers had landed at Palembang in Sumatra, which fell to an amphibious assault three days later.

When ABDACOM was dissolved on February 25, 1942, only Java remained to complete the Japanese program of conquest. The Allies’ desperate attempt to intercept the Japanese invasion fleet was defeated in the seven-hour Battle of the Java Sea on February 27, in which five Allied warships were lost and only one Japanese destroyer damaged. The Japanese landed at three points on Java on February 28 and rapidly expanded their beachheads. On March 9 the 20,000 Allied troops in Java surrendered. In the Indian Ocean, the Japanese captured the Andaman Islands on March 23, and began a series of attacks on British shipping. After the failure of ABDACOM, the U.S.–British Combined Chiefs of Staff placed the Pacific under the U.S. Joint Chiefs’ strategic direction. MacArthur became supreme commander of the Southwest Pacific Area, which comprised the Dutch East Indies (less Sumatra), the Philippines, Australia, the Bismarck Archipelago, and the Solomons; and Admiral Chester W. Nimitz became commander in chief of the Pacific Ocean Areas, which comprised virtually every area not under MacArthur. Their missions were to hold the U.S.–Australia line of communications, to contain the Japanese within the Pacific, to support the defense of North America, and to prepare for major amphibious counteroffensives.

Japan’s initial war plans were realized with the capture of Java. But despite their military triumphs, the Japanese saw no indication that the Allies were ready for a negotiated peace. On the contrary, it seemed evident that an Allied counterstroke was in the making. The U.S. Pacific Fleet bombed the Marshall Islands on February 1, 1942, Wake Island on February 23, and Marcus Island (between Wake and Japan) on March 1. These moves, together with the bombing of Rabaul on February 23 and the establishment of bases in Australia and a line of communications across the South Pacific, made the Japanese decide to expand so as to cut the Allied line of communications to Australia. They planned to occupy New Caledonia, the Fiji Islands, and Samoa and also to seize eastern New Guinea, whence they would threaten Australia from an air base to be established at Port Moresby. They planned also to capture Midway Island in the North Pacific and to establish air bases in the Aleutians. In pursuance of this new program, Japanese troops occupied Lae and Salamaua in New Guinea and Buka in the Solomon Islands in March 1942 and Bougainville in the Solomons and the Admiralty Islands (north of New Guinea) early in April.

Something to raise the Allies’ morale was achieved on April 18, 1942, when 16 U.S. bombers raided Tokyo—though they did little real damage except to the Japanese government’s prestige. Far more important were the consequences of the U.S. intelligence services’ detection of Japanese plans to seize Port Moresby and Tulagi (in the southern Solomons). Had these two places fallen, Japanese aircraft could have dominated the Coral Sea. In the event, after U.S. aircraft on May 3, 1942, had interfered with the Japanese landing on Tulagi, U.S. naval units, with aircraft, challenged the Japanese ships on their circuitous detour from Rabaul to Port Moresby. On May 5 and 6 the opposing carrier groups sought each other out, and the four-day Battle of the Coral Sea ensued. On May 7 planes from the Japanese carriers sank a U.S. destroyer and an oil tanker, but U.S. planes sank the Japanese light carrier Shoho and a cruiser; and the next day, though Japanese aircraft sank the U.S. carrier Lexington and damaged the carrier Yorktown, the large Japanese carrier Shokaku had to retire crippled. Finally, the Japanese lost so many planes in the battle that their enterprise against Port Moresby had to be abandoned.

“Secret of Shangri-La,”Pathé Gazette newsreel detailing the bombing raid on Tokyo led by U.S. Army Lieut. Col. James H. Doolittle on April 18, 1942.Stock footage courtesy The WPA Film Library

In the Battle of the Coral Sea (May 1942), U.S. naval airplanes thwart Japanese plans to occupy Port Moresby, New Guinea. From “The Second World War: Allied Victory” (1963), a documentary by Encyclopædia Britannica Educational Corporation.Encyclopædia Britannica, Inc.

Despite the mixed results of the Battle of the Coral Sea, the Japanese continued with their plan to seize Midway Island. Seeking a naval showdown with the remaining ships of the U.S. Pacific Fleet and counting on their own numerical superiority to secure a victory, the Japanese mustered four heavy and three light aircraft carriers, two seaplane carriers, 11 battleships, 15 cruisers, 44 destroyers, 15 submarines, and miscellaneous small vessels. The U.S. Pacific Fleet had only three heavy carriers, eight cruisers, 18 destroyers, and 19 submarines, though there were some 115 aircraft in support of it. The Americans, however, had the incomparable advantage of knowing the intentions of the Japanese in advance, thanks to the U.S. intelligence services’ having broken the Japanese Navy’s code and deciphered key radio transmissions. In the ensuing Battle of Midway, the Japanese ships destined to take Midway Island were attacked while still 500 miles from their target by U.S. bombers on June 3. The Japanese carriers were still able to launch their aircraft against Midway early on June 4, but in the ensuing battle, waves of carrier- and Midway-based U.S. bombers sank all four of the Japanese heavy carriers and one heavy cruiser. Appalled by this disaster, the Japanese began to retreat in the night of June 4–5. Though the U.S. carrier Yorktown was sunk by torpedo on June 6, Midway was saved from invasion. In the Aleutians, the Japanese bombed Dutch Harbor effectively and on June 7 occupied Attu and Kiska.

Midway, Battle ofIn June 1942, one month after the Battle of the Coral Sea, U.S. naval airplanes stopped the advance of the Japanese Imperial Navy near Midway Island. From “The Second World War: Allied Victory” (1963), a documentary by Encyclopædia Britannica Educational Corporation.Encyclopædia Britannica, Inc.

The Battle of Midway was probably the turning point of the war in the Pacific, for Japan lost its first-line carrier strength and most of its navy’s best trained pilots. Henceforth, the naval strengths of the Japanese and of the Allies were virtually equal. Having lost the strategic initiative, Japan canceled its plans to invade New Caledonia, Fiji, and Samoa.

The Chinese front and Burma, 1941–42

Japan’s entry into war against the western Allies had its repercussions in China. Chiang Kai-shek’s government on December 9, 1941, formally declared war not only against Japan (a formality long overdue) but also, with political rather than military intent, against Germany and Italy. Three Chinese armies were rushed to the Burmese frontier, since the Burma Road was the only land route whereby the western Allies could send supplies to the Nationalist Chinese government. On January 3, 1942, Chiang was recognized as supreme Allied commander for the China theatre of war; and a U.S. general, Joseph W. Stilwell, was sent to him to be his chief of staff. In the first eight weeks after Pearl Harbor, however, the major achievement of the Chinese was the definitive repulse, on January 15, 1942, of a long-sustained Japanese drive against Ch’ang-sha, on the Canton–Han-k’ou railway.

Chinese line the streets of K'un-ming as the first supply convoy reaches the city over the Allied-controlled Burma-Ledo road during World War II.

Thereafter, Chiang and Stilwell were largely preoccupied by efforts to check the Japanese advance into Burma. By mid-March 1942 two Chinese armies, under Stilwell’s command, had crossed the Burmese frontier; but before the end of the month the Chinese force defending Toungoo, in central Burma between Rangoon and Mandalay, was nearly annihilated by the more soldierly Japanese. British and Indian units in Burma fared scarcely better, being driven into retreat by the enemy’s numerical superiority both in the air and on the ground. On April 29 the Japanese took Lashio, the Burma Road’s southern terminus, thus cutting the supply line to China and turning the Allies’ northern flank. Under continued pressure, the British and Indian forces in the following month fell back through Kalewa to Imphāl (across the Indian border), while most of the Chinese retreated across the Salween River into China. By the end of 1942 all of Burma was in Japanese hands, China was effectively isolated (except by air), and India was exposed to the danger of a Japanese invasion through Burma.

Since the U.S. bombers that raided Tokyo on April 18 flew on to Chinese airfields, particularly to those in Chekiang (the coastal province south of Shanghai), the Japanese reacted by launching a powerful offensive to seize those airfields. By the end of July they had generally achieved their objectives.